Gas Productivity Enhancement by Wettability Alteration of Gas-Condensate Reservoirs

Abstract

Abstract Gas condensate reservoirs experience significant productivity losses as reservoir pressure drops below the dewpoint due to condensate accumulation and the subsequent reduction in gas relative permeability. One potential way to overcome this problem is to alter reservoir wettability to gas-wetting to reduce condensate accumulation in the near-wellbore and maintain high productivity. The aim of this study was to evaluate the effectiveness of various chemical treatments in altering wettability of gas-condensate reservoirs from liquid wetting to intermediate gas wetting. Coreflood experiments were conducted on carbonate and sandstone reservoir cores and Berea cores at simulated reservoir conditions. Several chemicals (fluorochemical and silane) were screened in this study to determine their capability in removing the trapped condensate from cores, enhancing gas relative permeability, and delaying condensate accumulation. The results of coreflood tests showed that the effectiveness of fluorochemical surfactant is affected by treatment volume, aging time, core permeability and temperature. Sandstone cores treated with 1.25 wt% silane chemical showed repellency to liquids (water and condensate) and an enhancement (up to 42%) in gas relative permeability. It was found that core permeability plays a role in wettability alteration agents' effectiveness. Wettability tests showed that contact angle on treated cores is 116° for water and 114° for condensate, indicating wettability alteration from liquid to intermediate gas wetting. Environmental Scanning Electron Microscope (ESEM) analysis performed on silanes-treated cores gave a conclusive evidence of wettability alteration at the pore scale. Introduction Maintaining production from gas reservoirs is a challenge due to the dropout of condensate and accumulation in the near wellbore region when the bottom hole flowing pressure decreases below the dewpoint pressure. Condensate flows along with the gas phase in the reservoir only when its saturation reaches or exceeds the critical condensate saturation. Gas well productivity starts to decline as condensate bank forms around the wellbore area. Without pressure maintenance (for example, gas cycling), condensate banking cannot be prevented. It may be delayed by using the proper exploitation methods, such as fracturing and horizontal completion.1–7 Condensate can be mobilized from the near wellbore region by either reducing capillary pressure or increasing drawdown pressure (viscous forces). Capillary pressure can be reduced by either decreasing the interfacial tension, wettability alteration. Solvents have been used to remove condensate banking from gas wells by decreasing interfacial tension and increasing liquid vaporization rate, but their effectiveness is temporary due to solvent flowback.8,9 Since most of reservoirs are preferential liquid wet, altering their wettability using chemical treatments to intermediate or gas wet reduces condensate entrapment and helps to maintain or increase gas production. Achieving this objective, chemicals are required to have certain properties such as longevity, non-damaging, thermally stable and low cost.